<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {}
        
        .form {
            padding-top: 10px;
            text-align: center;
            background-color: green;
            width: 500px;
            height: 300px;
            border-radius: 2%;
        }
        
        .form_text {
            margin-top: 30px;
        }
        
        .form_width {
            width: 100px;
        }
        
        .login_btn {
            color: white;
            background-color: blue;
        }
    </style>
</head>

<body>

    <form class="form">
        <h3>账号登陆</h3>
        <p>用户名:<input type="text"></p>
        <p>密码: <input type="password" maxlength="20" minlength="6"></p>
        <p><input type="submit" value="登录" class="login_btn form_width"></p>
    </form>
</body>

</html>